This invention relates to the production of cross sectional images of a body from measurements made using ionizing rays in general and more particularly to an improved method and apparatus for providing improved contrast in images generated in this manner.
Scanners have recently been developed by means of which a body may be scanned by moving a radiation source and radiation receiver parallel to the cross sectional plane of the body. In some models parallel displacement of the radiation source and the radiation receiver becomes unnecessary, when a fanshaped beam of rays with a multiplicity of radiation receivers is used. The heavily focused rays pass through the body in a cross sectional plane in such a manner that at least some rays always intersect in an image element. In known methods, the distribution of the attenuation coefficients in the cross sectional plane of the body can be derived by a convolution method through correction of measured data. In such a method the measurement data is convolved with a convolution kernel whereafter additive superposition of the convolved measurement data is carried out and reprojection of the image then accomplished. The measured values are converted into electrical signals and evaluated in an electronic computing system.
In one known method for producing a cross section image of the body, an X-ray or gamma-ray source furnishes a beam of nearly parallel rays which penetrates the body to be examined in the cross section plane and is absorbed by the body to a certain extent. Behind the body to be examined, the radiation falls on a detector. By displacing the radiation source and the detector parallel, step by step, the body is scanned sequentially in the cross section plane. Thereupon, the radiation source and the detector are tilted at a predetermined angle to an axis perpendicular to the cross section plane and the cross section plane of the body is again projected on the detector by parallel displacement. The radiation therefore passes through the individual image elements in a different direction. If this process is repeated several times, each element of the body in the cross section plane is imaged as many times as the system is tilted about the axis at a predetermined angle. The conversion of these different individual exposures of the cross section image to be produced of the body is obtained by means of an electronic computer, into which then, for instance, 28,000 equations with 6,400 variables are set. The cross section is first calculated by the electronic computer as a two-dimensional field of numbers, and subsequently, the numerical values of the individual image elements can be converted and displayed as a picture which is easy to interpret or can also be printed out by a printer (German Offenlegungschrift No. 1,941,433).
In this known method, the numerical reconstruction is arranged so that the pictures produced faithfully resemble the original cross section image, i.e. the numerical values in the individual image elements represent, with the exception of measurement or equipment errors, the unchanged radiation attenuation coefficients in the cross section plane of the object, averaged over a small area.
If the direction of radiation is fixed, the radiation source and the receiver go successively through discrete positions to scan the object in the cross section plane with a set of parallel ray beams. The direction of radiation is varied through a range of 180.degree. in individual angular steps of say 1.degree. . The measured values of the radiation attenuations of a single measurement series determined by parallel displacement are subsequently corrected point by point. After this correction, each measured value represents the sum of the radiation attenuation coefficients over the strip region covered by the measuring beam. In the second step, the corrected individual measurement series is convolved with a predetermined, permanently stored convolution kernal. In the third step, an intermediate image is produced from the convolved measurement series by re-projection. In the re-projection, every value of the series is uniformly distributed in the image plane over the strip region which is assigned to it according to the ray penetration geometry. In the last step, all intermediate images belonging to all radiation directions are summed by simple superposition to form the cross section image of the body (IEEE Transactions on Nuclear Science, June 1974, Vol. NS-21 no. 3 pages 46-49 and pages 59-63).
With this known method, cross sectional images of a body with numerical values in the individual image elements true to the original, except for measurement and method errors are also obtained. In practice, however, not only are such faithfully reproduced cross section images desired, but constrasting filtered cross sectional images are also desireable.